Membrane technology is an emerging field with annual growth rates of more than 10% (Nunes and Peinemann, 2001). It covers a wide variety of techniques. In dialysis and electrodialysis, separations take place under influence of a gradient in concentration or in electrical potential respectively. Pervaporation typically involves the phase transition of a liquid into a vapour during the passage through the membrane. The driving force of this partial pressure driven separation is maintained by the presence of a sweep gas or a vacuum at the permeate side. Pressure driven operations comprise microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and gas separation. Pressures in gas separation can rise up to 100 bar. When liquids are used in the feed, the nomenclature is defined rather arbitrarily. In general, MF is referring to processes running at pressures between 0.1 and 2 bar, UF between 1 and 5 bar, NF between 5 and 20 bar, RO between 10 and 100 bar. NF and RO are sometimes referred to as hyperfiltration as well (Mulder, 2003). An enormous variety of membranes can be applied in these processes, ranging from very dense to highly porous structures, with either an organic, inorganic or organo-mineral composition. Moreover, the selective top-layer can be combined with a variety of support layers to form optimized composite structures. Only for the fields of NF and RO, more than 250 patents and literature references can be found already before 1993 as reviewed by Petersen (1993) and still more than 100 to date (Vankelecom, De Smet, Gevers, Jacobs, 2004).
In order to select the most appropriate membrane for a given separation problem, many industrially available membranes need to be screened, which is often a time consuming and both labor and energy intensive process. Common lab-scale membrane screening takes place with either cylindrically shaped membranes (hollow fibres, capillary and tubular membranes) or membrane coupons cut from flat sheet membranes. They have to be sealed in the testing device, the feed has to be loaded and permeation has to run for several minutes, hours or even days, depending on the specific feed/membrane combinations. Moreover, since many different process parameters influence the membrane behavior, this testing should optimally be performed under a wide range of conditions for a given feed/membrane combination. Temperature, pH, type and concentration of solute, pressure, solvent choice, stirring speed, . . . are some examples of important parameters that define such conditions.
Being an emerging technology, an optimal membrane is not yet available for each specific separation problem, thus leaving an important incentive for the development of new membranes. Membrane synthesis typically takes place in a multi-parameter space, leaving again a whole set of prepared membrane to be tested. Phase inversion is a typical way to prepare membranes for pressure driven applications. It involves the transformation of a polymer solution into a solid polymer membrane, typically by immersing the polymer solution in a non-solvent bath. In this process, every parameter change can induce a significant change in membrane performance: organic or inorganic additives can be mixed in the polymer solution and the coagulation bath, different solvents can be selected, evaporation conditions can be varied (forced flow, gas atmosphere), casting temperature can be changes, . . . . Another typical synthesis process involves dip coating in which a support layer is dipped into a polymer solution that will form the actual separating top-layer. Polymer concentration, casting solvent, support pre-treatment, . . . are some of the main parameters involved in this process.
It is clear that membrane technology would benefit enormously from the possibility to perform the screening of membranes in a more efficient way than this sample by sample lab-scale testing.
The present invention provides a system allowing the high throughput screening of the performance of selected membranes for a given purpose. This system comprises multiple individual units and each unit comprises all features, which are typically present in a full-scale operational membrane filtration unit. In consequence, the data derived from the screening using the system of the present invention can be extrapolated to predict the behaviour of a given membrane in a full-scale system.